A web-offset printing press includes an inking assembly for each color of ink used in the printing process. Each inking assembly includes an ink reservoir as well as a blade disposed along the outer surface of an ink fountain roller. The amount of ink supplied to the roller train of the press and ultimately to a substrate such as paper is adjusted by changing the spacing between the edge of the blade and the outer surface of the ink fountain roller. The blade is divided into a plurality of blade segments, and the position of each blade segment relative to the ink fountain roller is independently adjustable by movement of an adjusting screw, or ink key, to thereby control the amount of ink fed to a corresponding vertical strip or zone of the substrate. Note that electronic equivalents of an ink key also exist, such as the equivalent disclosed in U.S. Pat. No. 5,129,320, issued Jul. 14, 1992 to Fadner. The term "ink key" is intended to include any device which controls the amount of ink fed to a corresponding vertical strip or zone of the substrate.
Typically, ink is also spread laterally from one vertical zone to adjacent vertical zones due to the movement of vibrator rollers, which oscillate in a lateral direction relative to the substrate.
The amount of ink on the ink fountain roller itself is also adjustable by changing the angle that the ink fountain roller rotates each stroke. Typically, this occurs by adjusting a ratchet assembly, as is known in the art.
In order to preset the initial positions of the ink keys, it is common for a printing press operator to examine printed copies or proofs of the image to be printed and to note the amount of color necessary in respective vertical zones of the image to be printed. Based on this visual examination as well as experience with the press, ink, and type of substrate (typically paper), the operator will preset the ink keys to approximate the settings that will be required once the press is running. As an example, low-tack yellow ink has a low pigment strength and requires a greater amount of ink to produce an image with a given optical density. As another example, uncoated paper requires more ink than does coated paper to achieve an image having a given optical density.
In addition to presetting the ink keys, once the press is running, it is common for a press operator to continually monitor the printed output and to make appropriate ink key adjustments in order to achieve appropriate quality control of the color of the printed image. For example, if the color in a zone is too weak, the operator adjusts the corresponding ink key to allow more ink flow to that zone; if the color is too strong, the corresponding ink key is adjusted to decrease the ink flow. During operation of the printing press, further color adjustments may be necessary to compensate for changing press conditions, or to account for the personal preferences of the customer.
The above-described visual inspection techniques used in connection with ink key presetting and color control are inaccurate, expensive, and time-consuming. Further, since the required image colors are often halftones combined with other colors, such techniques also require a high level of operator expertise.
Various other methods for presetting the ink keys are also known, which rely on a more exact measurement of plate coverage to achieve more accurate results than those obtained by visual estimation. Plate coverage is the ratio of the inked area to the total plate area, and provides a measure of the amount of ink required to print the desired image. By dividing the printing plate into zones corresponding to the ink keys, and determining the plate coverage of each zone, an initial ink key setting can be determined.
By way of example, U.S. Pat. No. 3,958,509 describes a method wherein the plate coverage of each ink key zone is determined by photo-electronic scanning of a portion of the printing plate. The ink key positions are calculated to be proportional to the plate coverage in a corresponding zone. U.S. Pat. Nos. 4,210,818, 4,187,435, and 4,180,741 also describe systems wherein ink keys are adjusted according to the plate coverage in a corresponding zone. Here, the plate coverage is determined using a light source to illuminate an image area such as a photographic film of the image or a printing plate to be printed and using light sensors to measure the light reflected from the film. Again, the ink key positions are calculated to be proportional to the plate coverage in a corresponding zone.
Other ink key preset systems are described in U.S. Pat. Nos. 5,170,711, 5,070,784, and 5,524,542. The systems described therein also account for various empirical parameters associated with the characteristics of the press, ink, type of job, and type of paper. These empirical parameters are typically learned from consistent usage of the preset system. Several different sets of parameters are often necessary for various types of printing jobs, especially for different plate coverages.
The above-described methods for presetting the ink keys are also somewhat inefficient and inaccurate because they do not take into account various factors such as ink back-flow, lateral ink spread, and ink saturation. Since substrate material is wasted until acceptable color is achieved, a more accurate method of presetting the ink keys will minimize the required number of adjustments while the press is running, saving both valuable time and material costs. Especially for print jobs of short duration, start-up waste can be a major percentage of total time and materials required.
Methods other than visual inspection of the printed image are also known for monitoring color quality once the press is running. These methods typically include measuring the optical density of a printed image. Optical density of various points of a printed image can be measured by using a densitometer or scanning densitometer either off-line or on-line of the web printing process. Off-line optical density measurements are performed by illuminating a test image with a light source and measuring the intensity of the light reflected from the image. Optical density (D) is defined as: EQU D=-log.sub.10 (R)
where R is the reflectance, or ratio of reflected light intensity to incident light intensity. An effective on-line method for accurately measuring the optical density of a printed image is described in U.S. patent application Ser. No. 08/434,928, invented by John C. Seymour, Jeffrey P. Rappette, Frank N. Vroman, Chia-Lin Chu, Bradly S. Moersfelder, Michael A. Gill and Karl R. Voss, and assigned to the assignee of the present invention.